The first generation of MEMS (micro-electromechanical systems) wavelength selective switches used single axis tiltable mirror arrays (one mirror per wavelength) to arbitrarily switch any set of optical wavelength signals incident at an input port to any of N output ports. A typical configuration was to disperse the wavelengths in a first axis (y) and switch in the orthogonal axis (x). Optimizing wavelength channel shape requires a tight beam waist in the y-axis at the MEMS mirror plane, while optimizing the number of achievable ports within a limited MEMS tilt range leads to a large beam waist in the x-axis. It is therefore advantageous to have a mirror array with an x-axis dimension significantly larger than y-axis dimension.
In order to achieve “hitless switching” (i.e. avoid scanning through intermediate ports), a 2-axis tilt is required for each mirror. A 2D gimbal arrangement that can fit within the footprint of the mirror can be used to produce an arrangement with a high fill factor. The 2D gimbal can be placed at the center of the mirror, with the disadvantage that the optical beam can not be centered on the mirror and does not fully utilize the available area. A hidden gimbal approach has been reported as a means to achieve the 2-axis tilt while maximizing the usable mirror area.
A significant challenge for both versions of 2D gimbals is that control of x and y axis tilt is not independent. This is due to coupling between drive electrodes as a result of shared electrostatic cavities. This coupling leads to a requirement for careful 2D calibration and control in order to follow the required “hitless” path trajectory. As a result, the switch path is broken into many smaller steps, at a cost to calibration time and to switching time.